Resistance to the action of insulin (i.e., insulin resistance) is a key pathogenic feature of type 2 diabetes. Even without diabetes, insulin resistance is a major risk factor for cardiovascular disease and early death. The two major causes of insulin resistance are obesity and genetic predisposition. We recently discovered that serum retinol binding protein (RBP4) concentrations are elevated in insulin resistant mice and humans. In a series of animal models we demonstrated that increasing serum RBP4 causes insulin resistance and glucose intolerance, whereas lowering serum RBP4 improves insulin sensitivity and glucose tolerance. Therefore, RBP4 may play a causal role in insulin resistance. Increased expression of RBP4 in adipose tissue may be partly responsible for elevated serum RBP4 in insulin resistant states. However, other factors may regulate serum concentrations of RBP4. The serum protein, transthyretin, binds RBP4 and plays a critical role in stabilizing RBP4 in serum. Treating mice with a drug (fenretinide) that blocks binding of RBP4 to transthyretin lowers circulating RBP4 levels, improves insulin sensitivity, and lowers blood glucose. Therefore, factors that determine how strongly RBP4 binds transthyretin may be important in the development of insulin resistance. Preliminary studies indicate that transthyretin levels are elevated in insulin resistant mice and in humans. Transthyretin is known to undergo several different post-translational modifications that could potentially influence its binding affinity with RBP4. Preliminary findings indicate that the pattern of these modifications is altered in obesity and insulin resistance. The hypothesis to be tested is whether transthyretin concentrations or post- translational modifications regulate serum RBP4 concentrations and/or RBP4 action in insulin resistant states. Both in vitro and in vivo studies in mice are proposed to evaluate the role of transthyretin in regulating RBP4. Insulin resistance is a major cause of type 2 diabetes and cardiovascular disease. Recent work in our laboratory has identified serum retinol binding protein (RBP4) as a potential cause of insulin resistance in humans and in mice. Insulin resistance is associated with increased levels of RBP4 in the circulation. Increased production of RBP4 in fat appears to be an important source of increased circulating RBP4 in humans and in mice. However, the serum protein, transthyretin, also plays a role in stabilizing RBP4 in circulation. The proposed work will study the interaction between transthyretin and RBP4 in terms of their biochemical interactions, metabolic effects in cultured cells, and metabolic effects in mice.